How do I programmatically determine if an object is a 'linked duplicate' of another?
Object.data
Linked duplicates will have the same linked data, if there is linked data.  For an empty ob.data is None
import bpy
from collections import defaultdict

scene = bpy.context.scene

data_links = defaultdict(list)
for ob in scene.objects:
    data_links[ob.data].append(ob)
    
for k, v in data_links.items():
    print(k, v)

Sample output,  notice there are 4 objects using the mesh &quot;Cube.001&quot;
&lt;bpy_struct, Mesh(&quot;Cube.001&quot;)&gt; [bpy.data.objects['Cube'], bpy.data.objects['Cube.001'], bpy.data.objects['Cube.002'], bpy.data.objects['Cube.003']]
&lt;bpy_struct, PointLight(&quot;Lamp&quot;)&gt; [bpy.data.objects['Lamp']]
&lt;bpy_struct, Camera(&quot;Camera&quot;)&gt; [bpy.data.objects['Camera']]
None [bpy.data.objects['Empty'], bpy.data.objects['Field']]